The present invention relates to a cell separation device using competition between electrostatic force and acoustic radiation force.
It is in general known that a cell has a negative charge on its surface and the charge is gradually reduced by a decrease in its living activity. For example, in an electric field erythrocytes are subjected to electrophoresis toward an anode caused by the negative charge of their surface. At this time, the mobility of the erythrocytes is not uniform and different depending on the surface condition of the respective erythrocytes. The causes thereof are thought to be different freshness of the respective erythrocytes and relevance with disease symptoms. The change in the charge which erythrocytes have were introduced in, for example, Donon D et al., Ann N Y Acad Sci 1983; 149-458, "Surface Charge of Old, Transformed, and Experimentally Deteriorated Erythrocytes", or Bartosz G et al., Mech. "Ageing Dev January 1984: 24(1) 1-7 Aging of the Erythrocyte. XIX. Decrease in Surface Charge Density of Bovine Erythrocytes."
It has been known since the 19th century that particles in a fluid can be trapped without contact by ultrasonic irradiation. Concerning the acoustic radiation force which particles receive when the radiation force acts on the particles, for example, in Acoust. Soc. Am. 89(1991) pp. 2140-2143, J. Wu, reported that he succeeded in trapping polystyrene spheres of 270 .mu.m diameter at the focal point of focal ultrasound. As regards the principle that particles are trapped by acoustic radiation force, in Acoustica 5 (1955) pp. 167-178, K. Yosioka and Y. Kawasima reported that they calculated the intensity, in a perfect fluid, of the acoustic radiation force which particles receive in a standing wave and a traveling wave, and the acoustic radiation force that the particles floating in the standing wave receive is in proportion to the volume of the particles and the frequency of the ultrasound forming the standing wave. Furthermore, Japanese Patent laid open No. 7-47259, proposed by the present inventors, discloses a manner of introducing ultrasound into a tube in which a fluid is allowed to flow so as to focus particles continuously within some area, or a method for collecting the focused particles. Furthermore, Japanese Patent laid open No. 6-241977 by the inventors discloses a particle separation device for separating and collecting particles having different particle sizes and made of different materials by the competition acoustic radiation force and other non-contact force such as electrostatic force.
It has been known heretofore that as the frequency of ultrasound used in a standing wave is gradually changed, the position of nodes of the standing wave changes accordingly and particles also move. In J. Acoust. Soc. Am. 91(1992), pp. 3152-3156, T. L. Tolt et al. reported a means for moving and concentrating actual particles trapped in nodes of a standing wave by sweeping, upwards and downwards, the frequency of ultrasound introduced into a fluid wherein the particles are dispersed. Furthermore, U.S. Pat. No. 5,225,089 by E. Benes et al. discloses a means for concentrating particles by raising the frequency of ultrasound radiated from an ultrasound source arranged in a channel.
Moreover, it has also been known heretofore that the position of nodes of a standing wave can be controlled by controlling the phases of ultrasound radiated from a plurality of different ultrasound sources for generating a superimposed waveform ,for example, as proposed by the present inventor Yasuda et al. in U.S. patent application Ser. No. 08/745,656. U.S. Pat. No. 4,743,361 by C. J. Schram discloses a means of applying this technique actually to measure physical properties of particles by observing how much the particles follow the movement of the position of nodes of a standing wave. It has also been known heretofore that when ultrasound having slightly different frequencies are radiated oppositely, the position of nodes of a generated standing wave advances by the slight difference between the frequencies.